Solar Flare Surprise

Dec.
15, 2008: Solar flares are the most powerful explosions
in the solar system. Packing a punch equal to a hundred million
hydrogen bombs, they obliterate everything in their immediate
vicinity. Not a single atom should remain intact.

At
least that's how it's supposed to work.

"We've
detected a stream of perfectly intact hydrogen atoms shooting
out of an X-class solar flare," says Richard Mewaldt
of Caltech. "What a surprise! These atoms could be telling
us something new about what happens inside flares."

The
event occurred on Dec. 5, 2006. A large sunspot rounded the
sun's eastern limb and with little warning it exploded. On
the "Richter scale" of flares, which ranks X1 as
a big event, the blast registered X9, making it one of the
strongest flares of the past 30 years.

NASA
managers braced themselves. Such a ferocious blast usually
produces a blizzard of high-energy particles dangerous to
both satellites and astronauts. Indeed, moments after the
explosion, radio emissions from a shock wave in the sun's
atmosphere signaled that a swarm of particles was on its way.

An
hour later they arrived. But they were not the particles researchers
expected.

NASA's
twin STEREO spacecraft made the discovery: "It was a
burst of hydrogen atoms," says Mewaldt. "No other
elements were present, not even helium (the sun's second most
abundant atomic species). Pure hydrogen streamed past the
spacecraft for a full 90 minutes."

Next
came more than 30 minutes of quiet. The burst subsided and
STEREO's particle counters returned to low levels. The event
seemed to be over when a second wave of particles enveloped
the spacecraft. These were the "broken atoms" that
flares are supposed to produce—protons and heavier ions such
as helium, oxygen and iron. "Better late than never,"
he says.

Above:
STEREO particle counts on Dec. 5, 2006. The vertical axis
measures the angle to the sun. Note how the initial hydrogen
burst arrived from a narrow angle while the ions that followed
swarmed in from all directions. The "swarming action"
is a result of deflections by the sun's magnetic field--a
force not felt by the neutral hydrogen.

At
first, this unprecedented sequence of events baffled scientists,
but now Mewaldt and colleagues believe they're getting to
the bottom of the mystery.

First,
how did the hydrogen atoms resist destruction?

"They
didn't," says Mewaldt. "We believe they began their
journey to Earth in pieces, as protons and electrons. Before
they escaped the sun’s atmosphere, however, some of the protons
recaptured an electron, forming intact hydrogen atoms. The
atoms left the sun in a fast, straight shot before they could
be broken apart again." (For experts: The team believes
the electrons were recaptured by some combination of radiative
recombination and charge exchange.)

"Simple,"
says Mewaldt. "Ions are electrically charged and they feel
the sun's magnetic field. Solar magnetism deflects ions and
slows their progress to Earth. Hydrogen atoms, on the other
hand, are electrically neutral. They can shoot straight out
of the sun without magnetic interference."

Imagine
two runners dashing for the finish line. One (the ion) is
forced to run in a zig-zag pattern with zigs and zags as wide
as the orbit of Mars. The other (the hydrogen atom) runs in
a straight line. Who's going to win?

"The
hydrogen atoms reached Earth two hours before the ions,"
says Mewaldt.

Mewaldt
believes that all strong flares might emit hydrogen bursts,
but they simply haven't been noticed before. He's looking
forward to more X-flares now that the two STEREO spacecraft
are widely separated on nearly opposite sides of the Sun.
(In 2006 they were still together near Earth.) STEREO-A and
–B may be able to triangulate future bursts and pinpoint the
source of the hydrogen. This would allow the team to test
their ideas about the surprising phenomenon.

"All
we need now," he says, "is some solar activity."

For
more information about this research, look for the article
"STEREO Observations of Energetic Neutral Atoms during
the 5 December 2006 Solar Flare" by R. A. Mewaldt et
al, in a future issue of the Astrophysical Journal Letters.